Problem: On the first 3 quizzes of her history class, Nadia got an average score of 85. What does she need on the next quiz to have an overall average of 87?
Explanation: Let her score on the next quiz be $x$ The sum of all of her scores is then $3 \cdot 85 + x$ The same sum must also be equal to $4 \cdot 87$ Solve: $x = 4 \cdot 87 - 3 \cdot 85 = 93$.